In order to operate in the ice infested waters of the arctic and sub-arctic, offshore platforms must be able to resist forces which act on the platforms as a result of wind, water waves, water currents, and moving ice masses. The forces acting on an offshore platform which are a result of such environmental effects are hereinafter collectively referred to as "Environmental Loads." The outer hull of an offshore platform must be designed so that it can withstand the local action of the Environmental Loads on the outer hull which may develop as a result of ice masses or water waves impinging on the hull. Additionally, an offshore platform must have a large enough base contact area with the sea floor foundation and enough mass to prevent slippage between the base and the foundation and to prevent the bearing capacity of the foundation from being exceeded, and to prevent an overturning moment which develops as a result of the Environmental Loads from overturning the platform.
In the past, it was believed that the major Environmental Loads acting on such a platform resulted from moving ice masses impinging on the structure. Therefore, in the past the limiting factor in the design of an offshore structure was whether it could resist the Environmental Loads which resulted from the impingement of large moving ice masses on its outer hull. This resulted in massive offshore structure designs which were designed to resist the Environmental Loads which were believed to result from the impingement of large ice masses on an offshore structure.
For example, U.S. Pat. No. 4,422,804 to Gerwick, Jr., et al. discloses an offshore structure which is designed to be operated in arctic waters where icebergs may be present. Gerwick, Jr., et al. disclosed a platform that has an array of vertically extending scallop shaped ballast compartments which form the outer periphery of the platform. The platform generally has the shape of a cylinder and is preferably constructed of concrete. While the platform design disclosed by Gerwick, Jr., et al. will be effective for operating in arctic ice infested waters, it is believed that it is overbuilt for the actual Environmental Loads which will result from ice masses that may be encountered in most offshore areas. Further, the generally cylindrical shape of the platform will increase the Environmental Loads which will act on the platform as a result of water waves.
Another example of a design that may be used in arctic waters where large moving ice masses may be encountered is disclosed in U.S. Pat. No. 4,639,167 to Petty, et al. The platform disclosed in Petty, et al. has external walls that am angled away from the vertical. These sloped external walls were intended to bend the ice from the horizontal and make it ride up the walls until it broke in flexure. While this design may work for failing ice in flexure, it is believed that this type of design will also result in a platform that is improperly sized for the actual Environmental Loads which will result from ice masses that can be expected to be encountered in most offshore areas.
What is desired is an offshore structure design that Will withstand the Environmental Loads that result from moving ice masses, but that will minimize the Environmental Loads that are a result of other environmental effects such as water waves and water currents. This will result in a design that is more economical and easier to build than currently existing offshore structure designs intended for ice infested waters.